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Accrued maintenance liability
 12 Months Ended
Dec. 31, 2020
Accrued Maintenance Liability [Abstract]  
Accrued maintenance liability Accrued maintenance liability
Movements in accrued maintenance liability during the years ended December 31, 2020 and 2019 were as follows:
Year Ended December 31,
20202019
Accrued maintenance liability at beginning of period$2,190,159 $2,237,494 
Maintenance payments received345,699 736,423 
Maintenance payments returned(412,492)(352,032)
Release to income upon sale(95,042)(249,187)
Release to income other than upon sale(344,210)(207,849)
Lessor contribution, top ups and other66,281 25,310 
Accrued maintenance liability at end of period$1,750,395 $2,190,159